1. Field of the Invention
The present invention relates to a light scanning apparatus, in particular, to a light scanning apparatus having a multi beam scanning optical system that simultaneously scans multiple light beams, and also to a latent image forming apparatus and an image forming apparatus which use the light scanning apparatus.
2. Description of the Related Art
Conventionally, a light scanning apparatus is known that performs latent image formation by focusing a light beam emitted from a light source such as a laser diode (LD) or the like to form a latent image on a surface to be scanned of an image carrying member (for example, a photoreceptor having photoconductive properties) via a scanning optical system including a light deflection device (for example, a light deflector of a polygon mirror or the like). The light scanning apparatus is used as a latent image forming device of an image forming apparatus of an electrophotographic system (for example, a digital copier, a laser printer, a laser facsimile, a hybrid machine or the like combining these functions). In addition, as image forming apparatuses gain higher speed and higher density, there is proposed a light scanning apparatus using a multi beam scanning optical system that simultaneously performs writing of multiple lines in a sub scanning direction.
In a photoreceptor used as an image carrying member in an image forming apparatus of an electrophotographic method, even when a total exposure energy density provided to the photoreceptor is the same, there is a phenomenon of reciprocity failure in which the state of latent image formation differs if the relationship between light quantity and exposure time differs. That is, exposure of an insubstantially short time in comparison to exposure taking a comparatively long time has less change in quantity of electrical potentials of the photoreceptor despite the total exposure time being equal and reciprocity failure is generated.
This is because when the light quantity is strong, recohesion quantity of a crier increases so that a quantity of the carrier reaching a surface decreases. And, in the case of a multi-beam scanning optical system, this appears as an image concentration irregularity.
Here, FIG. 9 is an example of using a 4ch LDA (4 channel laser diode array) with 4 LDs from LD1 to LD4 arranged thereby as a scanning optical system of the image forming apparatus. A boundary area of the LD1 and the LD2 is virtually simultaneously exposed so that the area is hit by a strong light quantity in a short period of time. Comparatively, in a boundary area of the LD4 and the LD1, first, the LD4 is exposed. Thereafter the LD1 is exposed so that a time lag is generated and consequently, the area is hit by a weak light quantity in a long period of time. In this case, latent image electrical potential distribution is formed deeply in the exposure with the time lag and a toner becomes easily attachable. As a result, the boundary area between the LD4 and the LD1 has an image concentration thicker than other parts so that an image concentration irregularity is generated.
The phenomenon of reciprocity failure as described above depends, among the property values of a photoreceptor, for example, on the membrane thickness of a charge generation layer (CGL) of an organic photoreceptor, the degree of movement of carrier, the quantum efficiency and the carrier generation quantity. Therefore, it is desirable to provide an image forming system in which reciprocity failure hardly occurs and which is configured to include a photoreceptor and a scanning optical system. But, by a conventional measuring method, a space resolution capability of only about a few millimeters can be obtained so that a precision sufficient for analyzing the mechanism cannot be obtained. Therefore, the most appropriate exposure condition can only be determined from an output image.
Here, as an example of conventional technologies, in JP2004-77714A, a task is set to obtain a high quality image without generating any image quality defection due to reciprocity failure even when multi-beam scanning is performed and it is described that “by adopting jump over scanning, in the case any pair of adjacent scanning line is chosen, a scanning number j of each can be necessarily set to a differing combination so that the time of scan interval can be set to greater or equal to one main scanning time.” As a result, image quality defection due to banding by reciprocity failure can be drastically reduced and an image that is hardly recognizable as image quality defection in practical use can be obtained.
With regard to the conventional technologies described in JP2004-77714A, it is recognized that the phenomenon of reciprocity failure influences images, but for the above-described reason, a latent image formation mechanism is not analyzed so that light quantity emitted to a photoreceptor and the specific setting quantity of exposure energy density are not taken into consideration.